1. Field of the Invention
The present invention relates to water-swellable, water-insoluble polymeric materials. Specifically, the present invention concerns a porous structure formed from a water-swellable, water-insoluble polymeric material.
2. Background of the Related Art
Water-swellable, water-insoluble polymeric materials, commonly known as superabsorbents, are known for use in a variety of applications. For example, it is known to incorporate such water-swellable polymeric materials into absorbent products such as, diapers, sanitary napkins, bandages, adult incontinence products and the like.
When water-swellable polymeric materials are incorporated into personal care products such as diapers, the water-swellable polymeric materials are often present within and carried by a fibrous web. The fibrous web carrying the super absorbent material forms the absorbent core of such personal care products. The water-swellable polymeric materials are present within the fibrous web in order to increase the absorbent capacity of the fibrous web. In this manner, a smaller absorbent core can be employed in the personal care product since the composite of fiber and water-swellable polymer has a greater absorbency per unit volume. Unfortunately, water-swellable polymeric materials are generally substantially more expensive than the fibrous webs in which they are located. Accordingly, in order to minimize the cost of the absorbent core, it is generally desirable to utilize the water-swellable polymeric material to its greatest extent and to employ a water-swellable polymeric material having the greatest possible capacity while still maintaining other desirable properties such as, gel strength, the ability to absorb and retain fluid against an applied pressure, and the like.
Attempts have been made to increase the water absorbing properties of water-swellable polymeric materials to improve their performance and thereby, minimize the amount of water-swellable polymer present in an absorbent structure. Such an attempt is described by Masamizu et al. in U.S. Pat. No. 4,742,086, issued May 3, 1988. Masamizu et al. describes a process for manufacturing porous polymer and the porous polymer formed by the process. The process described by Masamizu comprises the steps of:
1. Forming an O/W emulsion wherein the inner phase of the O/W emulsion is a hydrophobic phase and the outer phase thereof is a water phase containing at least one water soluble polymerizable monomer;
2. Adding the O/W emulsion to a hydrophobic dispersing medium containing an oil-soluble surfactant to form an O/W/O emulsion; and
3. Polymerizing the monomer. The processes is described as producing a polymer having a porous structure.
As used by Masamizu et al. the term "porous" refers to a polymer having internal voids which internal voids are not connected to the surface of the polymer particles. The outer surface of the polymer particles has a pitted appearance. The type of porous particles formed by Masamizu et al. is clearly described in FIGS. 2 and 3 of Masamizu et al. The porous particles described by Masamizu et al. are claimed to have superior initial water-absorbing speed and improved capacity under pressure.
Similarly, U.S. Pat. No. 4,795,762 issued Jan. 3, 1989 to Diamantoglou, et al. is directed to a water-swellable composition having a spongy structure. The composition is described as having a high swelling capacity and swelling rate.
U.S. Pat. No. 4,703,067 issued Oct. 27, 1987 to Mikita et al. is directed to a process for preparing a dry, solid water absorbing polyacrylate resin. The process described by Mikita et al. involves forming a monomer mixture of potassium acrylate, a polyvinyl monomer, and water, said monomer mixture comprising 55 to 80 weight percent monomers based on total monomer mixture weight. The monomer mixture may contain up to 15 weight percent of a non-aqueous solvent having a boiling point between 40.degree. and 150.degree. C. A polymerization initiator is added to the monomer mixture and the mixture polymerized while using the exothermic heat of reaction to drive water away from the acrylate resin.
The solvents described by Mikita et al. may be water miscible, e.g., ethanol, methanol and the like, or water immiscible, e.g., benzene, toluene, tetrahydrofuran, and the like. The polyacrylates produced when the nonaqueous solvents are present are found to possess quicker absorption rates but not to possess higher absorption capacities at 15 minutes, see for example Tables 6 and 7. Mikita et al. does not describe the physical characteristics of the polyacrylate resin formed by the described process.
The formation of porous particles of polymeric material is known in connection with non water-swellable polymeric materials. For example, U.S. Pat. No. 4,611,014 issued Sep. 9, 1986 and U.S. Pat, No. 4,612,334 issued Sep. 16, 1986, both patents being issued to Jones et al., are directed to porous polymers. The polymers described by Jones et al. are highly porous cross-linked functionalized polymers having interconnected cavities and a pore volume greater than 5.6 cubic centimeters per gram. The polymers are generally based on styrene and/or acrylate compositions. The porous polymers described by Jones et al. have interconnected cavities or chambers of micron dimensions.